Radiative Transfer in a Turbulent Expanding Molecular Envelope: Application to Mira

Mira (o Ceti), the prototypical Mira variable, due to its proximity (D = 128⁺¹⁹₋₁₄ pc, van Leeuwen et al. 1997) is one of the best-studied red giant stars. Mira has an extended molecular envelope created by mass-loss, whose properties can be determined from studies of rotational emission lines from various molecules at millimeter and submillimeter wavelengths. The CO molecule is a particularly important probe because its has a relatively high abundance relative to hydrogen, [CO/H₂] > 10⁻⁴, which is constant throughout large parts of the envelope, from the stellar photosphere, R_(star) = 2.5 × 10¹³ cm, out to a radius R ~ 5 × 10¹⁶ cm. The bulk of the emission observed in a given rotational line arises from a limited certain part of the envelope, so that multi-transition studies allows us to determine temperature, density, and velocity profiles throughout the envelope, as well as an estimate of the mass-loss rate.